We have demonstrated that a mouse genetic locus governing expression of immunoglobulin light chain polymorphisms is closely linked to loci determining cell surface antigens (Lyt-2 and Lyt-3) which mark functional subpopulations of T-cells. A major emphasis of this proposal is to obtain plasmacytomas which produce light chains bearing these polymorphisms for structural analysis and for production of specific antisera, and also to analyze the genetics of the chromosomal region bearing these genes. It is apparent that this region is involved in a major way with immune function. We also will perform structural analysis of the Lyt-2 and Lyt-3 alloantigens to investigate for possible relatedness to immunoglobulins and obtain some hint of possible function. In addition, we will investigate the development of mouse T-lymphocytes during gestation. We have developed an antiserum specific for cells present in the fetal thymus early in gestation but not late, and we will use this reagent in immunochemical and immunofluorescence studies to explore the ontogeny of the immune and hemopoietic systems.